This invention relates to glass structures such as glass fibers in which the surface characteristics of the galass structure have been modified to enable the glass fibers, in strand form, to be chopped without losing their integrity, while possessing other favorable characteristics. Some of the other favorable characteristics possessed by the chopped strands include: flowability of the chopped strands during processing, mixing, handling, conveying and molding within a resinous matrix; low bulk density; heat resistance; lightness of color; and the chopped strands import high impact strengths to resinous matrices due to a strong bonding relationship between the sized chopped strands and the resinous materials, whether thermoset or thermoplastic materials.
Difficulties in the establishment of a chopped glass strand that possesses integrity during processing, flowability during processing, lightness in color and which imparts high impact strengths to the resinous matrices are well known in the art.
From the time of formation of glass fibers to the more distant point in time of their incorporation into a resin matrix to reinforce the same, many processing operations will have had to be carried out. Immediately after the glass fibers are formed and traveling at linear speeds in excess of 10,000 feet per minute a protective coating is applied to the glass fibers to prevent mutual abrasion. Subsequently the sized fibers are gathered onto a rotating collection package or routed directly to a chopping apparatus where the glass strands are chopped into lengths ranging from about 1/8 to about 2 inches, or even longer lengths.
When the strands are gathered onto a package it is preferable to dry the package prior to positioning the package on a creel with numerous other packages, in order to form a roving which is collected on a roving doff. When the strands are fed directly to the chopper, the drying may be prior to or subsequent to chopping. When the strands are dried prior to being chopped a less integral strand results whereas when the strands are dried subsequent to chopping a highly integral strand results. Because of the differences in integrity the amount of solids of the sizing on the glass fibers may be adjusted accordingly to compensate therefor. Subsequent to chopping, the chopped strands may be either packaged for a later use or be combined and mixed with a resinous material to form a premix which is used as a molding compound. Finally the molding compound may be either packaged for subsequent use or may be immediately used in a molding operation to form reinforced articles.
The treatment applied to the glass fibers at forming must be multifunctional for the purposes of this invention. It must be capable of protecting the individual fibers from mutual abrasion especially when the fibers in the form of a strand, are combined to form a roving and the roving is subsequently wound onto a collection package. The treatment must also be capable of maintaining the glass strands making up the body of the roving in an integral unit before, during, and after chopping, so that during handling, conveying, mixing, and molding, the chopped strands have flowability and are further capable of a strong bonding relationship with a resinous matrix that is to be reinforced.
Difficulties in the establishment of a strong and permanent bonding relationship between the surfaces of glass fibers and a resinous material have in general become well known in the art. Because of the nonporous character of glass fibers, as distinguished from a high degree of porosity available in natural fibers such as the fibers of cellulose, wool, cotton, hemp and the like, penetration of resinous materials into the fibers is not available for use in establishing a bonding relationship between such glass fibers in a resinous material. Because glass fibers naturally form into elongate rods having very smooth surfaces, as distinguished from the rough surface characteristics of natural fibers, a gripping relationship or a mechanical bonding is difficult to establish between resinous materials and the untreated glass fiber surfaces. Thus a physical anchorage of the type relied upon chiefly for the establishment of a bonding relationship between natural fibers and resinous materials is not capable of being developed with glass fibers. Glass fibers may be etched or roughened to present a surface of some porosity but desirable strength characteristics of the glass surfaces are simultaneously lost.
In the absence of the ability to make use of physical forces in bonding, it becomes necessary to rely upon the development of a relationship requiring chemical bonding or physical-chemical forces based upon molecular or ionic attraction and the like. With synthetic resinous fibers e.g. nylon, polyester, etc. a strong bonding relationship can be developed with the smooth surfaces because such fibrous materials are resinophilic in character and therefore preferentially receptive to resinous treating materials. In addition, the resinous materials, of which the fibers are formed, have the ability of being softened by heat or solvent in a manner to enable the development of a desired bonding relationship with the applied treating material. Such chemical forces resulting from the softening of the synthetic fiber surfaces are not available with glass fibers because the glass fibers are inert to heat solvents and because the glass fiber surfaces are dominated by groups that are hydrophilic in character and therefore receive moisture in preference to resinous materials. As as result, only a weak bonding relationship is capable of being established in the first instance and even this limited bonding is reduced in the presence of moisture or high humidity sufficient to cause a moisture film to form and separate the resinous coating from the glass fiber surfaces with a moisture interface.
When a strong bonding relationship cannot be established between glass fibers and a resinous material used in combination therewith, maximum utilization of the strength properties of the glass fibers connott be made available in the products that are formed. Even where a fair bonding relationship between glass and resin can be established under extremely dry conditions, the strength properties of the glass fiber reinforced plastic composite depreciates greatly under high humidity conditions or in the presence of moisture.
When glass fibers are formed into strands, containing many fibers, and the strands are subsequently chopped into lengths of preferably from about 1/8 inch to about 1 inch it is desirable to have the chopped strands possess integrity. That is, after chopping it is desired to have a strand in a rod-like manner without the many fibers making up the strand separating from the rod-like structure. The desirability of this rod-like structure is important when a resinous matrix is to be reinforced with glass fibers to improve strength and other characteristics. Another desirable characteristic of the chopped strands of this invention is that they have a high degree of flowability during processing, especially within the resinous matrix that is to be reinforced so that the chopped strands have a uniform dispersement within the matrix and not be heavily grouped in one local concentration and void of chopped strand in another concentration. Still another desirable characteristic of the chopped strands of this invention is that they possess a high degree and uniform quality of whiteness, so that when the strands are incorporated into a resinous matrix, there is substantially no fiber prominence, i.e. it is difficult to determine visually and physically, such as by touch, that the resinous matrix is reinforced with glass fibers.
Difficulties in the establishment of a reinforced article having little or no fiber prominence have in general become well known in the art. Fiber prominence may be attributed to discoloration of the treatment on the fiber when compared to the color of the resin matrix that is reinforced and may also be attributed to the physical surfacing of the fiber to the resin surface. Fiber prominence is an undesirable characteristic in reinforced articles especially glass reinforced articles. It is thought that the discoloration of the fibers occurs during the curing of the reinforced article. Many reinforced articles do not have pigment coloration therein and for this reason, in the past, undesirable fiber prominence characteristics have been a problem.
It is therefore an object of this invention to produce a treatment for glass fibers and to produce glass fibers treated with a material to enable the glass fibers, in strand form to be formed into a roving and subsequently chopped wherein the chopped strands maintain integrity during further processing.
It is another object of the invention to provide a new and improved coating for glass fibers so that the coated fibers when gathered into a strand, chopped, and subsequently used as a chopped reinforcement in resinous matrices, remain flexible and substantially insoluble in the matrices.
It is yet another object to produce glass fibers, which when incorporated with a resinous matrix, exhibits substantially no fiber prominence within the matrix.
It is yet another object to produce glass fibers, which when chopped, possess good flowability characteristics during processing.
It is still another object to produce glass fibers, which when incorporated in a resinous matrix, exhibits a strong bonding relationship with the matrix.
Further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates from the following description.
Processability of the glass strands becomes extremely important during the introduction of the treated strands to the chopper. It is desirable to obtain chopped strands of uniform length, but this becomes difficult when the chopper becomes clogged with previously chopped fibers. Static forces are set up on chopping and must be combated.
A lack of strand integrity during processing is more than a problem. It is detrimental to the uniform distribution of chopped strands within a resinous matrix because the strands conglomerate or clump together. When a thermosetting matrix is to be reinforced, a premix, comprising the chopped strands in the resin, is formed. When a thermoplastic matrix is to be reinforced, the chopped strands and resin are introduced into an injection molding machine as a dried blend via vibration. If filamentation of the chopped strands occurs, the strands will tend to stick together through physical forces or static forces, and cause a non-uniform distribution of the strands into the matirx, or a non-uniform distribution of the strands into the injection molding machine.
The degree of integrity possessed by the chopped strands becomes extremely important when the strands are incorporated with a resinous matrix. During the incorporation it is desirable to obtain some filamentation of the strand sufficient to increase the surface area of available reinforcement, but insufficient to be incapable of actual reinforcement. It has been found that when the strands have no degree of filamentation upon incorporation with a resinous material, strengths of the composite are low. The same phenomenon is present when there is no integrity of the chopped strand after incorporation of the strand with the resinous matrix. Therefore, a compromise between a highly integrated strand and a highly filamentized strand must be reached.
Chemical as well as physical forces contribute to the degree of filamentation of the treated strand, after incorporation into a resinous matrix.
The inventive treatment, hereinafter described in greater detail provides all of the advantages as above described.